BBFC Passed Anime Updates

If it is as transphobic as you claim the bbfc would have some kind of description on the back.
Something does not have to depict a literal hate crime or a direct "I am being bigoted towards you because you are this identity" moment to be bigoted though, and that's all the BBFC will care about in their rating. In this case it's a villain character who is a huge caricature that makes trans women out to be hypersexual predators and plays it for laughs, similarly to how a lot of 20th century media depicted gay people although far more explicit since Konosuba ain't a subtle anime. As someone who has their life directly affected by transphobia, I knows it when I sees it.
Yeah from what I remember it was rather questionable on that front sadly :( I haven't seen the series, did you like that? :)
The series in terms of theme is kind of a send up of the isekai genre, and in terms of humour is about what you would expect from a comedy harem series or something. It is what it is. The movie is just a lot more...ick.